Projection lens arrangements for focusing a projected image on a remote viewing surface are well known in the prior art. Such lens arrangements include those utilized with front and overhead projectors, and still and motion picture video projectors.
For example, consider the projection lens arrangement in a conventional overhead projector. In such a projector, the lens is mounted above and spaced-apart from the stage of the projector. A transparency or computer controlled liquid crystal panel for providing an image to be projected is positioned on the stage. The distance between the transparency or object and the entranceway to the projection lens is referred to as the object length and is about 15 inches in length in some overhead projectors. A Fresnel lens arrangement causes light, emitted from a high intensity lamp disposed below the stage, to be directed upwardly into the projection lens at an angle. This angle is called the field coverage angle and is about 18 degrees. For the purpose of focusing the image to be projected onto a remote viewing surface, the overall length of the projection lens arrangement is adjustable. This overall length is referred to as the vertex length of the lens arrangement.
While the above-described projection lens arrangement has proven satisfactory in large bulky overhead projectors, such an arrangement can not be readily used in a small compact projector system, such as a compact projector system disclosed in copending U.S. patent application Ser. No. 08/059,550.
In the case of a small compact projector, the object length must be substantially shorter and thus, the field coverage angle must be substantially greater. However, by increasing the field coverage angle various aberrations can be introduced, such as field curvature aberrations and other types of known aberrations.
Therefore, it would be highly desirable to have a new and improved projection lens arrangement and method of using the arrangement which can be used readily in a small compact projector system. Such a new and improved projection lens arrangement would have a relatively short object length but yet a sufficiently narrow field coverage angle to enable optical compensation for eliminating or at least substantially reducing the effect of optical aberrations such as field curvature aberrations.
In order to focus a variety of different sized images to be projected onto a remote viewing surface, a projection lens arrangement must be variable for focusing purposes. In this regard, the vertex length of the lens arrangement must be variable but yet sufficiently small to enable the lens arrangement to be utilized in a small compact projector system.
However, shortening the vertex length introduces other problems. For example, by shortening the vertex length it is difficult, if not impossible to have sufficient variations to reach substantially all anticipated field coverage angles when the arrangement employs a relatively short object length.
Therefore, it would be highly desirable to have a new and improved projection lens arrangement that has both a relatively small variable vertex length and object length to enable the lens to be utilized in a small compact projector but yet a sufficiently long vertex length to permit focusing for substantially all anticipated field coverage angles.
Another problem associated with a lens arrangement having a short vertex length is that the spacing between the optical elements within the lens arrangement must necessarily be very short in distance. Thus, in order to reach substantially all anticipated field coverage angles in a relatively convenient manner, the focusing adjustments must be very precise and accurate.
Therefore, it would be highly desirable to have a new and improved projection lens arrangement which can be easily and automatically adjusted to focus an image on a remote viewing surface. Such a lens arrangement should be easily adjusted for focusing purposes, and relatively inexpensive to manufacture.